1. Field of the Invention
The present invention relates to an optical modulation device used for optical communication and light signal processing, and more particularly to a structure of an electro-absorption type light modulation device and a saturable absorption light-switching device having a semiconductor light waveguide channel structure.
2. Description of Related Art
A light modulation device having a semiconductor light waveguide channel structure generally has a P-i-N heterojunction structure where a first conductivity type semiconductor layer, a light waveguide layer, and a second conductivity type semiconductor layer are formed in this sequence on a semiconductor substrate. A reverse bias voltage is applied to the light waveguide layer from electrodes which are connected to the first conductivity type semiconductor layer and the second conductivity type semiconductor layer, respectively. By applying the reverse bias voltage, an electric field is applied to the light waveguide layer, and the light absorption factor in the light waveguide layer increases by the electro-absorption effect. The intensity of light guided into the optical modulation device is modulated by the applied reverse bias voltage.
Such an optical modulation device is disclosed, for example, in literature 1: xe2x80x9cHigh-speed InGaAlAs/InAlAs Multiple Quantum Well Optical Modulators with Bandwidths in Excess of 20 GHz at 1.55 xcexc, Isamu Kotaka, et al, IEEE Photon. Technol. Lett., Vol. 1, No. 5, pp. 100-101, 1989; and in literature 2: xe2x80x9cElectro-absorption Modulators on Semi-insulating InP Substrate for High Speed Modulationxe2x80x9d, Ajisawa et al., 1990 Institute of Electronic Information and Communication Engineering, Spring National Conference, C-239, p. 4-294.
In the structure of the optical modulator disclosed in the literature 1, an n-type cladding layer (n-InAlAs), a light waveguide layer (MQW), and a p-type cladding layer (p-InAlAs) are formed in this sequence on the n-type semiconductor substrate (n-InP) so as to create a P-i-N junction structure. A p-type ohmic contact layer is formed on the p-type cladding layer, and an n-type ohmic contact layer is formed on the back side surface of the n-type semiconductor substrate. A p-side electrode is connected to the p-type ohmic contact layer, and an n-side electrode is connected to the n-type ohmic contact layer. In other words, an electrode is formed at the top side surface and the back side surface of the optical modulator, respectively.
FIG. 1A is a cross-sectional view depicting an example of a configuration of a conventional plane parallel plate type optical modulation element, similar to the optical modulator disclosed in literature 1. FIG. 1B is an equivalent electric circuit diagram of a conventional plane parallel plate type optical modulation element. A double heterojunction structure part (which is called first junction structure part) 108, which comprises the first conductivity type cladding layer 102, the light waveguide layer 104, and the second conductivity type cladding layer 106, is formed on the first conductive substrate 100, and the second conductivity type side electrode 112 is formed on the second conductivity type cladding layer 106 via the second conductivity type ohmic contact layer 110. A first conductivity type side electrode 114 is formed on the back side surface of a substrate 100.
In FIG. 1B, a reference character RS shows the resistance of the double heterojunction structure part 108 which comprises the first conductivity type cladding layer 102, the light waveguide layer 104, and the second conductivity type cladding layer 106. A reference character Cj indicates the junction capacitance of the double heterojunction structure part 108. A reference character Rsub indicates the resistance of the first conductivity type substrate 100. A reference character Ce indicates inter-electrode capacitance or static capacitance between the second conductivity type side electrode 112, which is on the double heterojunction structure 108, and the first conductivity type side electrode 114 which is on the back side surface of the substrate 100.
In the structure of the optical modulator disclosed in the literature 2, the n-type etching stopper layer (n-InGaAs), the n-type cladding layer (n-InP), the light waveguide layer (i-lnGaAsP), and the p-type cladding layer (p-InP) are laminated in this sequence on the semiconductivity type semiconductor substrate, so as to form a P-i-N junction structure. The p-type ohmic contact layer (p-InGaAs) is formed on the p-type cladding layer. The layer above a part of the n-type cladding layer is formed in a stripe manner. Both sides of the stripes are buried by a high resistance insulating layer. Mesa is formed on the substrate by the striped structure and the insulating layer where both sides of the structure are buried. The p-side electrode is formed on the p-type ohmic contact layer, and the n-side electrode is formed on the n-type etching stopper layer, which is exposed from the mesa on the substrate.
However the above mentioned optical modulator has the following problems.
At first, in the case of the optical modulator disclosed in literature 1, the distance between electrodes is short since electrodes are plane parallel plates. Therefore the static capacitance is large. To implement an optical modulator which can operate faster, the static capacitance must be sufficiently small.
In the case of the optical modulator disclosed in the literature 2, the main heat generation source is a pair of the structure formed on the substrate. Therefore, when a substrate, where a light waveguide channel structure is formed, is mounted on a carrier or heat sink with the top surface of the substrate turned to the carrier or heat sink side to improve heat radiation of the optical modulator, the p-side electrode and the n-side electrode are normally bonded to the patterns, which are formed on the heat sink or on the carrier, so as to be electrically isolated from each other. However in the structure of the optical modulator disclosed in the literature 2, the p-side electrode and the n-side electrode are formed to have different heights, so the distance between the electrodes and the top surface of the heat sink or the carrier are different. More specifically, the p-side electrode formed on the top surface of the mesa is closer to the top surface of the heat sink or the carrier than the n-side electrode formed on the n-type etching stopper layer. Therefore, when the p-side electrode, which is closer to the top surface of the heat sink or the carrier, is bonded, a gap is generated between the n-side electrode and the heat sink or the carrier, and bonding the n-side electrode to the pattern is difficult. Since the contacting area between the element having a light waveguide channel structure and the heat sink or the carrier becomes small, a desired heat radiation may not be obtained. Since the contacting area, i.e., bonding area, is small, the adhesive strength between the element and the heat sink or the carrier may be weak. The electric resistance may increase because the n-side electrode cannot electrically contact the pattern formed on the heat sink or the carrier, or cannot make sufficient contact therewith. As a result, a desired long term reliability for the optical modulator may not be obtained.
It is an object of the present invention to provide a semiconductor optical function device which has better heat radiation than a prior art, excelling in long term reliability.
According to the present invention, the semiconductor optical function device comprises a first junction structure part which further comprises a first conductivity type cladding layer, a light waveguide layer, and a second conductivity type cladding layer, on a substrate, and a second junction structure part which is formed at a position on the substrate isolated from the first junction structure part, which includes the first conductivity type cladding layer and the second conductivity type sub-cladding layer. A first electrode is formed on the first junction structure part, and a second electrode is formed on the second junction structure part. Also, the heights from the top surface of the substrate to the top surface of the first electrode and the second electrode are substantially the same.
Therefore, if the device is mounted to the supporting element for heat radiation with the side where the first electrode and the second electrode are formed (top surface side of the substrate) at the side contacting the supporting element of heat radiation, the top surfaces of both the first electrode and the second electrode can be contacted to the supporting element.
Here, the supporting element for heat radiation may be a heat sink provided on the carrier or the carrier itself. In a part contacting the electrode of the supporting element for heat radiation, a pattern electrically separating the electrodes is provided. Therefore, the first electrode and the second electrode contact each pattern of the supporting element for heat radiation, respectively.
Since the heights of the top surfaces of the two electrodes are substantially the same, as described above, the contact status of the supporting element for heat radiation and the top surfaces of the two electrodes is good, and the contacting area between the supporting element of heat radiation and the electrode can be large. Therefore, the heat radiation of the device can be improved. Also, the adhesive strength between the electrode and the supporting element for heat radiation can be increased, so each electrode and the pattern provided on the supporting element can be electrically connected, and the electric resistance of the device is not increased. As a result, a desired long term reliability can be obtained.